For the production of so-called spunbond nonwoven webs it is generally known that a plurality of extruded fibers, after melt spinning and cooling, are drawn off collectively as a fiber curtain, stretched, and deposited on a deposit belt to form a nonwoven web. The deposited fibers result in a fiber formation within the nonwoven web which, among other things, determines the strength of the nonwoven web. In doing so, the nonwoven web strength is normally determined on a so-called MD axis, and on a so-called CD axis. The MD axis is also the running direction of the belt, in which the nonwoven web is continuously guided after the depositing of the fibers. The CD axis is orthogonal to the MD axis and describes the strength of the nonwoven web at a right angle to the running direction of the belt. Due to the moving during depositing, nonwoven webs of this type are predominantly formed with a depositing oriented on the MD axis. This results in a greater strength of the nonwoven web on the MD axis in relation to the strength on the CD axis. A ratio between MD and CD strengths lies in the region between 1.5 and 3.5, depending on the product setting and the polymer. For the production of technical products, however, spunbonded nonwoven webs having a nonwoven web strength that is distributed equally to the greatest extent possible are required. To influence the fiber formation there are, therefore, various apparatus and methods known in the prior art, for the purpose of obtaining a uniform strength of the nonwoven web to the greatest degree possible in the MD and CD axes.
For example, an apparatus and method for guiding and depositing synthetic fibers to form a nonwoven web is described in WO 2008/087193A2, in which the fiber curtain is drawn off by means of a drawing nozzle and subsequently guided as a fiber stream to a deposit belt. The fiber stream, formed by a primary airstream of the drawing nozzle and the fiber curtain, is conducted through guidance paths formed by a plurality of guidance means disposed in pairs. In this manner, acceleration and spreading of the fiber stream can be generated by means of cross-section changes and constrictions within the guidance pathways, which act in particular on the depositing of the fibers. In addition, air intake slits are formed above the guidance means, beneath the drawing apparatus, which enable the feeding of a secondary airstream. In this manner it is possible to influence the pressure differences in the guidance pathways.
With the known apparatus and with the known method, the depositing of the fiber stream is substantially influenced by means of changes of the cross-section within the guidance pathways and by resulting flow technical effects, in order to obtain a uniform fiber formation within the nonwoven web. However, in this manner it is only possible to influence the fiber stream in a flow direction within the guidance pathways.
A method and an apparatus is known from EP 1 340 842 A1 which shows a drawing apparatus formed by means of a drawing channel, which is connected directly to a cooling device for the fibers. The cooling airstream is used substantially in order to guide the fiber curtain through the drawing channel. A plurality of guidance means are associated with the surface of the drawing channel facing the deposit belt, which form a plurality of merging guidance pathways for the guidance of the fiber stream. The guidance means form numerous air intake slits, through which secondary airstreams are introduced. The guidance means form two merging diffusor-type guidance pathways, such that the fiber stream can be altered by means of cross-section changes and expansions. The depositing of the fibers is therefore also only possible by an influencing the fiber stream in the flow direction. In order to, nonetheless, obtain a distribution of the fibers during depositing that is uniform to the greatest degree possible, a suction device divided into numerous zones is associated with the deposit belt on the undersurface thereof. In this manner, it is possible to generate two different suction effects for receiving the fibers on the upper surface of the deposit belt, having, however, the substantial disadvantage that the fibers are deposited with different intensities and resulting in differing densities of the nonwoven web.